1. Field of the Invention
The present invention relates to an air-fuel ratio control device of an engine, and more particularly, it relates to an air-fuel ratio control device which executes feedback control at an air-fuel ratio on an excessively rich side in an industrial engine.
2. Description of Related Art
In a fuel supply system of an engine, an air-fuel ratio control device determines a fuel injection amount which realizes an optimum air-fuel ratio, on the basis of data of a detected engine operation state, to supply a fuel to the engine via fuel supply means such as a fuel injection valve, whereby feedback control of the air-fuel ratio is usually executed.
FIG. 8 shows an example of a constitution of an air-fuel ratio control device 1E which executes such an air-fuel ratio feedback system. A suction tube pressure sensor 15 disposed in a suction passage 3, and an engine rotation number sensor 14, an engine temperature sensor 13 and the like disposed for an engine 2 input detection signals into an electronic control unit 10C. The control unit calculates a fuel injection amount which realizes a target air-fuel ratio on the basis of various pieces of information of these signals, and outputs the amount as a fuel injection valve driving signal to a fuel injection valve 5 to control the air-fuel ratio.
However, it is known that an error is generated between the target air-fuel ratio and an actual air-fuel ratio owing to various factors such as product fluctuations, deterioration with time, and disturbance. Therefore, for the purpose of eliminating this error, an O2 sensor is disposed in an exhaust gas passage as in an air-fuel ratio control device disclosed in JP-A-7-208139, or an air-fuel ratio sensor 12 is disposed in an exhaust gas passage 4 as in the air-fuel ratio control device 1E described above and disclosed in JP-A-10-288075, whereby control is executed on the basis of a detected actual air-fuel ratio in many cases.
FIG. 9 shows a control block diagram by such an air-fuel ratio control device. The device multiplies basic injection pulse information Tp calculated on the basis of a suction air amount (C1) by a correction amount a calculated on the basis of air-fuel ratio information A/F by an air-fuel ratio sensor or an O2 sensor (C4), multiplies the result by other correction amounts K1 and K2 or adds the amounts, and outputs the result as a final pulse width Ti to the fuel injection valve 5, thereby executing the feedback control of the air-fuel ratio.
FIG. 10 is a graph showing output characteristics of the O2 sensor and the air-fuel ratio sensor. In the O2 sensor, a voltage noticeably changes around a theoretical air-fuel ratio, and the air-fuel ratio sensor exhibits nearly linear characteristics in accordance with the air-fuel ratio. However, in a car in which an exhaust gas is purified by using a ternary catalyst, control is executed around the theoretical air-fuel ratio at which a catalyst purification efficiency is highest. Therefore, the target air-fuel ratio can be realized by using either of the O2 sensor and the air-fuel ratio sensor.
On the other hand, in an industrial engine having a comparatively small displacement for use in a power generator, a lawn mower or the like, for request from a cost aspect in addition to the purposes of protecting the engine and acquiring safety of the engine, a catalyst purification device is not used but an air-fuel ratio is controlled so as to be on an excessively rich side from a theoretical air-fuel ratio, thereby decreasing nitrogen oxides in an exhaust gas. However, at the excessively rich side air-fuel ratio, an O2 sensor cannot be used owing to a problem of sensitivity, and an air-fuel ratio sensor having linear output characteristics is used, so that a steep rise of system cost is incurred.